The present invention is a cycle monitoring system and method that acquires, logs and analyzes analog and/or digital signals from component sensors in a process control plant such as a nuclear power plant. The system continuously monitors the sensors and records steady-state and transient phenomena. Compression of the data is performed to reduce storage load. The compression ratio for transient data is driven by a threshold indicating a measurement signficant to stress determination. Transient data is retained at a higher sampling rate, so that the parameters of transient such as the maximum value can be determined for stress analysis. The stress analysis determines pressure and thermal stresses which are used to calculate a usage factor which indicates the stress age and thus the remaining life of a component. The system provides estimates of fatigue accumulation associated with selected locations of critical power plant components. Measurements of plant process and response parameters are used to provide data for calculation of fatigue accumulation. The system includes a computer containing a database that includes thresholds and response characteristics of the plant components to varying types of stress. The process required to calculate fatigue for monitored components and locations uses Duhamels integral to calculate transient stress, combines the transient stress with other stresses to produce a usage factor from stress ranges designating when a stress cycle has occurred. The system can be used to plan routine maintenance, repair and replacement decision, and to justify alternative plant operation modes and life extension.